The invention relates to a multiphase venturi flow metering method. More particularly, the invention relates to a method for measuring fluid flow characteristics in a multiphase fluid stream containing liquid and gaseous phases which passes through a gradiometer-venturi flow meter.
Venturi flow meters are widely used to measure the flow rate in a flowline. UK patent application GB 2261519 and International patent application PCT/GB93/00885 disclose venturi flow meters for monitoring the rate of flow of a water and oil containing production stream in an oil production well.
European patent application 0684458 discloses the use of a pair of axially separated venturi flow meters for measuring flow rate of oilwell effluents containing hydrocarbons, gas and water.
In this and the other known devices the density of the fluid mixture is measured by measuring the pressure difference in a vertical or inclined gradiometer section of the well where there is no significant flow restriction and the venturi flow meter(s) is or are placed in an adjacent section. Since the pressure difference in the venturi flow meter is proportional to xcfx81v2 and the mixture density xcfx81 is known the fluid velocity and volume flow rate can be calculated. The aforementioned European patent application 0684458 specifies that the twin venturi flow meter arrangement may be equipped with fluid homogenisation means and that if the densities of the aqueous, oily and gaseous components are known also the fluid composition can be determined by periodic variation of the fluid velocity through the twin venturi flow meter arrangement and by cross-correlating the pressure drops measured across the venturis.
Other multiphase flow monitoring systems which address the problem of slip between the fluid phases but which do not employ a venturi are known from U.S. Pat. Nos. 3,909,603 and 5,361,206.
The article xe2x80x9cDesign of a flow metering process for a two-phase dispersed flowxe2x80x9d published by C. Boyer and H. Lemonnier in the International Journal Multiphase Flow Vol. 22, No. 4, pp. 713-732, 1996 (Elsevier Science Ltd.) discloses that in a two-phase venturi flow meter a velocity slip may occur between the gaseous and liquid phases at the throat of the venturi.
A conclusion derived in the article is that it is possible to determine a critical bubble or droplet diameter beyond which the homogeneous flow model is no longer valid.
The method according to the preamble of claim 1 is known from U.S. Pat. No. 4,856,344. In the known method it is assumed that the slip between the liquid and gas phase in a bubble flow regime remains constant over the length of the venturi and a fluid homogeniser is used since the measurement would be misleading if the fluid mixture would not be reasonably homogeneous.
The present invention aims to provide a multiphase venturi flow metering method which is able to operate accurately not only when a substantially homogeneous multiphase flow passes through the venturi but also when a substantially inhomogeneous multiphase flow, such as a slug flow, passes through the venturi.
The method according to the invention comprises:
measuring liquid holdup (xcex11,I) at or near the inlet of the venturi;
determining a slip factor (S) which expresses the difference between the gas and liquid velocity at a selected location in the venturi, which slip factor is based on the measured level of liquid holdup; and
calculating the fluid flow characteristics on the basis of an algorithm which takes into account the measured liquid holdup (xcex11,I) at the inlet of the venturi and different slip factors at the inlet (Si) and in the throat of the venturi (St).
Preferably, if the liquid holdup at the inlet of the venturi xcex11,I exceeds a pre-determined value slipfactors at the inlet of the venturi (Si) and in the throat of the venturi (St) are determined from empirical correlations, which slipfactors are used as input in a first algorithm, together with the pressure drop between the inlet and throat of the venturi, to calculate the superficial liquid velocity, whereas if the liquid holdup at the inlet of the venturi remains below the predetermined value then the slip factors Si and St are determined using empirical correlations given in a second algorithm.